Systems having modules with buffer chips

ABSTRACT

In some embodiments, the invention includes a system having first and second modules, the first module having a first group of chips and the second module having a second group of chips, and a circuit board including first and second module connectors to receive the first and second modules, respectively. The system also includes a first buffer on the first module and a second buffer on the second module, and a path including conductors in a first section that splits into a second section and third section, wherein the second section couples to the first buffer and the third section couples to the second buffer, and wherein impedances of the second and third sections are at least 50% greater than impedances of the first section.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present inventions relate to semiconductor chip modules and, moreparticularly, to layouts for paths for circuit boards and modules.

2. Background Art

Modules include circuit boards, such as printed circuit boards (PCBs),that have chips (integrated circuits) on one or both sides of themodules. Examples of memory modules include single in line memorymodules (SIMMs) and dual in line memory modules (DIMMs). The modules arepositioned on motherboards, which may also support a controller, such asa memory controller (which may be integrated with other chipsetfunctions or in a processor).

Traditional bus design, such as that used by current synchronous dynamicrandom access memories (SDRAMs) and double data rate (DDR) SDRAMs,involves the use of relatively long stubs (called a stubbed bus) asdescribed in the following example in which two modules and a controllerare on a motherboard. A bus of conductors extends a portion of themotherboard from the controller to the first and second modules. Whenthe bus is adjacent the first module, relatively long stubs from the busextend to the first module and to package connections of chips in themodule. Likewise, when the bus is adjacent the second module, relativelylong stubs from the bus extend to the second module and to packageconnections of chips in the second module. Package connections are usedto interface with the world outside the chip. Examples of packageconnections include pins and balls (such as for ball grid arrays or flipchip arrangements).

Electrical reflections occur as a result of the stub. Electricalreflections from relatively long stubs tend to slow the maximum rate atwhich voltage switches may occur. Some systems have been used with veryshort stubs extending to chips from busses in the modules.

Changes in impedance can cause undesirable reflections. When changes inimpedances are necessary, it may be desirable to increase or decreaseimpedances in steps. In the case in which a conductor splits into twosections, it is often desirable to raise the impedance of the twosections following the split to be about twice that of the sectionbefore the split. However, achieving this ratio is not always practicaland a lower ratio may be used.

A path typically at least one termination device (called a termination),such as a termination resistor, to allow electrical energy to dissipate.Terminations may be positioned on the device originating the signal (forexample, the controller), on the motherboard, on the module, on the die(chip), and/or in the package. Terminations are positioned between theconductor to be terminated and a reference voltage node, for example,having a ground voltage, a power supply voltage, or some other voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventions will be understood more fully from the detaileddescription given below and from the accompanying drawings ofembodiments of the inventions which, however, should not be taken tolimit the inventions to the specific embodiments described, but are forexplanation and understanding only.

FIG. 1 is a schematic plan view representation of a system including amotherboard, a controller, and two module connectors (not including themodules).

FIG. 2 is a schematic plan view representation of the motherboard ofFIG. 1 with the modules inserted in the connectors.

FIG. 3 is a schematic front view representation of a second module ofFIG. 2.

FIG. 4 is a schematic cross-sectional side view representation of asecond module of FIG. 3 taken along line 4—4.

FIG. 5 is a schematic front view representation of a first module ofFIG. 2.

FIG. 6 is a schematic cross-sectional side view representation of afirst module of FIG. 5 taken along line 6—6.

FIG. 7 is a schematic representation showing multiple lines and packageconnections which are represented by a single line and packageconnections in FIGS. 1-6.

FIG. 8 is a schematic representation of an alternative to FIG. 6.

FIG. 9 is a schematic representation of an alternative to FIG. 6.

FIG. 10 is a schematic representation of a termination package of FIGS.2 and 3.

FIG. 11 is a schematic plan view representation of a system including amotherboard, a controller, and two module connectors (not including themodules).

FIG. 12 is a schematic plan view representation of the motherboard ofFIG. 11 with the modules inserted in the connectors.

FIG. 13 is a schematic plan view representation of a system including amotherboard, a controller, and two module connectors (not including themodules).

FIG. 14 is a schematic plan view representation of the motherboard ofFIG. 13 with the modules inserted in the connectors.

FIG. 15 is a schematic cross sectional side view representation of thesystem of FIG. 14.

FIG. 16 is a schematic front view representation of module 1 of FIG. 14.

FIG. 17 is a schematic representation of impedances of the system ofFIG. 14.

FIG. 18 is a schematic partially plan and partially front view of asystem like that of FIG. 14 with the addition of error correction code(ECC) chips and buffer chips included on the modules.

FIG. 19 is a schematic representation of on die terminations in thebuffers of FIG. 18.

FIG. 20 is schematic representation of off die terminations for thebuffers of FIG. 18.

FIG. 21 is a schematic representation of the system of FIG. 14 includinga clocking path.

FIG. 22 is a schematic representation of the system of FIG. 21 includinga n additional clocking path.

FIG. 23 is a schematic plan representation of a system including amotherboard, a controller, connectors and modules inserted therein.

FIG. 24 is a schematic plan representation of a system including amotherboard, a controller, connectors and modules inserted therein.

FIG. 25 is a schematic representation of a routing path of the system ofFIG. 24.

FIG. 26 is a schematic representation of routing paths of the system ofFIG. 14.

FIG. 27 is a schematic cross sectional side view representation ofmodule 1 of FIG. 14 in which a short term card is used in place ofmodule 2.

FIG. 28 is a schematic plan representation of the short term card ofFIG. 27.

FIG. 29 is an alternative to FIG. 28.

FIG. 30 is a schematic cross sectional side view representation of analternative embodiment for the system of FIG. 14.

FIG. 31 is a schematic cross sectional side view representation of analternative embodiment for the system of FIG. 14.

FIG. 32 is a schematic representation of a selectively enabled on dietermination circuit that may be used in the systems of FIGS. 30, 31 and34.

FIG. 33 is a schematic representation of a selectively enabled on dietermination circuit which is an alternative to that of FIG. 32 and maybe used in the systems of FIGS. 30, 31 and 34.

FIG. 34 is a schematic representation of a system employing a layout ofFIG. 30.

FIG. 35 is a schematic plan view representation of a single sided systemsimilar to the dual sided system of FIG. 14.

FIG. 36 is a schematic cross sectional side view representation of asingle sided module.

FIG. 37 is a schematic cross sectional side view representation of asingle sided module.

FIG. 38 is a schematic representation of a system having three modulesand a controller.

FIG. 39 is a schematic plan view representation of a system including amotherboard, a controller, and two module connectors with modules.

DETAILED DESCRIPTION

The following detailed description describes multiple inventions whichare claimed in different patent applications. The same description isprovided in each application. However, the various inventions are notrestricted to the details of the figures or written disclosure. Indeed,the inventions may be practiced through details different from thoseshown in the figures and described herein.

1. Overview: A Simple Example

FIGS. 1-10 provide a schematic representation of a relatively simplesystem illustrating features of the inventions. Note that FIGS. 1-38 aresomewhat transparent (schematic) rather than being strictly plan, front,side, or cross-sectional views in that some objects below the surfaceare sometimes shown. Further, the figures are not intended to showcorrect relative sizes or shapes of objects in the drawings. FIG. 1illustrates a system 10 having a motherboard 12. A controller 14 isassociated with motherboard 12. Module connectors 20 and 22 are onmotherboard 12. Module slots 16 and 18 are formed in module connectors20 and 22. When modules 1 and 2 are inserted in the connectors, moduleslots 16 and 18 are filled. In the case in which memory modules are tobe inserted in module connectors 20 and 22, controller 14 includesmemory controller functions. (The inventions are not restricted to usewith memory modules.) Controller 14 may be in a chipset (e.g., a northbridge/hub) or it may be in a processor chip or group of chips orelsewhere.

A connector connection is an electrically conductive interface between amodule connector (e.g., module connector 20 in FIG. 1) and a conductoron a path. For example, conductor connections C1 are interfaces betweenmodule connector 20 and conductors on path 1. There are N parallelconnector connections in C1 and N parallel conductors in path 1. In thisdisclosure, connector connections are designated with the letter “C”followed by a number (e.g., C1, C2, etc.). Various devices may be usedfor connector connections including, for example, pin through holes,pads, and balls.

Modules typically include fingers, such as fingers 62 in FIG. 5. Thefingers interface with connector contacts on the module connectors. Theconnector contacts are designated with the letter “M” followed by anumber (e.g., M1, M2, etc.). “M” is chosen to indicate it interfaceswith the module. The connector contact may be a spring or other element.The inventions are not restricted to any particular connector contacts.As an example, in FIG. 5, finger 64 interfaces with a connector contactsM8 which is coupled to conductors 44.

A package connection is an electrical conductive interface between themodule and a chip package. In this disclosure, a chip includes a die anda package of some sort connecting the die to the outside world andperhaps also protecting the die. (Note that in common usage, the termchip is sometimes used synonymously with the term die.) There are avariety of ways in which the package connection may occur. Examples ofitems used in the package connections include pins and balls (such asfor ball grid arrays which may be used in flip chip arrangements).Package connections are designated with the letter “P” followed by anumber (e.g., P1, P2 etc.).

Routing path 1 and routing path 2 are paths that may be on the surfaceof and/or inside the motherboard. They may be data paths and may beuni-directional or bi-directional. Path 1 is shown with solid lines andpath 2 is shown with dashed lines. Paths 1 and 2 are in an arrangementthat may be referred to as a split ring. Clocking and control signalsare not illustrated in the simplified FIGS. 1-10. Path 1 extends fromcontroller 14 to connector connections C1 and connector contacts M1 onmodule connector 20. Note that there are N parallel conductors in path1. Accordingly, there are N connector connections designated asconnector connections C1 and N connector contacts designated asconnector contacts M1. The term “extend” does not imply that the path isin a straight line.

Path 1 continues at connector contacts M2. Note there is a gap betweenM1 and M2. As shown in FIG. 2, this gap is filled by conductors as shownon module 1 between fingers that interface with Ml and fingers thatinterface with M2. (In FIGS. 1 and 2, M1 and M2 are on opposite sides ofmodule connector 20, but that is not required. M1 and M2 could be on thesame side of module connector 20.) Path 1 continues from M2 to connectorconnections C2 to connector connections C3 and connector contacts M3.Path 1 continues from a connector contacts M4 to a connector connectionsC4. Again, there is a gap between M3 and M4 which is filed withconductors on module 2.

Likewise, path 2 (shown in dashed lines) extends from controller 14 toC5 to M5, from M6 to C6 to C7 to M7 and from M8 to C8. As shown in FIG.2, gaps between M5 and M6 and between M7 and M8 are filled withconductors on modules 1 and module 2 as shown. As with the case of path1, path 2 includes N parallel conductor lines and C5, C6, and C7 eachrepresent N connector connections. Likewise, M6, M7 and M8 eachrepresent N connector contacts on module connectors 20 and 22.

The conductors on Module 2 between M3 and M4 are referred to asconductors 58. With reference to FIGS. 2 and 10, conductors 58 includeconductors 58-1, 58-2, . . . 58-N. Termination package 30 includes Nterminations T1 T2, . . . TN (Ts). Terminations Ts are shown asresistors. In practice, they may be one or more transistors, discreteresistors, or other devices.

For high speed signaling, it is often desirable that there a groundconductor associated with every signal conductor on the motherboard, orthere is at least one ground conductor for every two signal conductors,although the inventions are not limited to either of these ratios.Referring to FIG. 10, the N conductors 58 each are terminated to aground plane, although the inventions are not so limited. Conductors(e.g., N or N/2) from the ground plane couple to connector connectionsC4 (of which there are, for example N or N/2). Conductors 32 (of whichthere are, for example, N or N/2) are at a reference node Vref, whichmay be power supply voltage, ground voltage or some other voltage.Termination package 42 and conductors 44 may be similar.

Since the reference lines 32 and 44 are used whether or not there are onmodule terminations, the on module terminations of termination packages30 and 42 reduce the number of connector contacts (M) and connectorconnections (C) by exactly or essentially N for each module for each twopaths. In the case of four paths, it would be reduced by exactly oressentially 2N per module. This results in exactly or essentially a ¼reduction in the number of connector contacts (M), correspondingfingers, and connector connections (C) for the paths (which may be datapaths).

FIGS. 3, 4, 5, and 6 provided additional details regarding theillustrated embodiment. Of course, the inventions are not restricted tothese details. Package connections P1 and P2 each represent N packageconnections on path 1 and package connections P3 and P4 each represent Npackage connections on path 2. In FIGS. 4 and 6, path 1 (called 60 onmodule 1) and path 2 are slightly set back from chip 26 and 38respectively. This is not required in practice, but is provided forconvenience in making the figures. Very short stubs 54 and 52 protrudefrom package connections P1 and P3 into chips 26 and 38, respectively.These stubs are extremely short in comparison to the stubs oftraditional stubbed busses. Note that in FIG. 3, the path 1 is shownwith front and back vertical lines for ease of illustration andunderstanding. In practice, the lines may be staggered as shown in FIG.3 or line up one behind the other.

FIG. 7 illustrates that conductors 60 include N conductors 60-1, 60-2 .. . 60-N. Short stubs 54 include N short stubs 54-1, 54-2 . . . 54-N.Package connections P1 include package connections P1-1, P1-2 . . .P1-N. In FIG. 7, chip 26 includes a die 26A and a package 26B.

The position of connector connections C1 and C2 are selected for ease ofillustration. In practice, they may be positioned more like that shownin FIG. 8 or 9. Further, as shown in FIG. 9, a chip may have more thanone set of package connections and short stubs (54 and 66) perconductor.

2. Examples With Additional Paths

As shown above, by using on module terminations, the number of connectorconnections, connector contacts, and corresponding fingers on themodules can be reduced by a factor of exactly or essentially ¼. Thefollowing examples of FIGS. 11-14 will apply this to a system with morechips. The number of module connections and corresponding fingers islimited by the pitch of the module connections and the length of theconnector. In standard computer motherboards, there is a certain amountof space for module connectors. Accordingly, for a given pitch andlength, the number of module connections and corresponding fingers islimited. The reduction of ¼ can be useful where connections and fingersare in shortage. The inventions are not limited to these details.

In FIGS. 11-17, bi-directional routing paths 1, 2, 3, 4, 5, 6, 7, and 8each include N conductors. Connector connections (C), connector contacts(M), and package connections (P) shown in FIGS. 11-17 represent Nconnector connections, connector contacts, and package connections.

FIG. 11 shows a system 90 without modules 1 and 2 and FIG. 12 showssystem 90 with modules 1 and 2. System 90 includes a motherboard 94 anda controller 92. Module connectors 116 and 118 are on motherboard 94 andinclude each a front side closer to controller 92 and a back sidefarther from controller 92. Modules 1 and 2 are inserted in module slots16 and 18 in connectors 116 and 118, respectively. Module 1 includes oneset of chips I1, I2 . . . I16 and module 2 includes another set of chipsI1, I2 . . . I16. Paths 1, 3, 6, and 8 are shown in solid lines andpaths 2, 4, 5, and 7 are shown in dashed lines. Path 1 includesconductors from controller 92 to C1 to M1 and from M5 to C5 to C14 andM14. As can be seen by comparing FIGS. 11 and 12, module 1 couples M1and M5 and module 2 couples M14 and “on module terminations” (OMT),which are terminations for path 1. Note that the OMT may be similar toor somewhat different from that of FIG. 10.

Path 2 includes conductors from controller 92 to C21 to M21 and from M13to C13 to C6 and M6. As can be seen by comparing FIGS. 11 and 12, module2 couples between M21 and M13 and module 1 couples M6 and an OMT, whichare terminations for path 2. The route of paths 3, 4 . . . 8 can be seenin FIGS. 11 and 12. The paths may be terminated on a single end (e.g.,merely the OMT) or may be terminated on dual ends (in OMT and also incontroller 92). The OMTs may be located in a variety of places (e.g.,FIGS. 16, 36, and 37).

A problem with the routing of system 90 is that modules 1 and 2 are notinterchangeable unless it does not matter if they are rotated 180degrees. This is because in module 1, the OMTs are coupled to chips I2,I4, I5, and I7, but in module 2, the OMTs are coupled to chips I10, I12,I3, and I15. If for some reason rotation is unobtainable, module 1 mustbe different than module 2, which causes additional expense. If errorcorrection code (ECC) chips are added to modules 1 and 2, the ECC chipshave to be in between chips I4 and I5 and I12 and I13 to keep rotationpossible. A disadvantage of having to rotate modules, is that they mightbe inserted in the wrong orientation. It may be preferred to have themodules and module slots keyed, so the modules cannot be inserted intothe module slots in the wrong orientation.

System 100 shown in FIGS. 13 and 14 solves this problem with FIGS. 11and 12. System 100 is like system 90 except for slightly differentrouting as shown in the figures and described as follows. In FIGS. 11and 12, path 1 goes between C5 and C14. By contrast, in FIGS. 13 and 14,path 1 goes between C5 and C18. Similarly, in contrast, in FIGS. 13 and14, path 3 goes between C7 and C20, path 6 goes between C10 and C21, andpath 8 goes between C12 and C23. In comparison with system 90, therouting of system 100 is referred to as a swizzle routing because thepoints of connector connection swizzles from the front side to the backside of module 2. Note that in system 100, the OMTs are in the sameplaces in module 1 and module 2. Therefore, module 1 could be insertedinto slot 18 and module 2 could be inserted into slot 16 and system 100would perform the same. Modules 1 and 2 and slots 16 and 18 can be keyedso they cannot be rotated. Therefore, in this example, only one type ofmodule needs to be on stock and it cannot be inserted the wrong way.

Note that system 100 provides only one example of a routing in which theOMTs are in the same position. A variety of other routings will providethe same result. For example, through an additional swizzle in moduleconnectors 116 and 118 could cause each OMT to switch from the front tothe back side of modules 1 and 2. The relative position of the OMTscould also be changed.

FIG. 15 provides a cross sectional representation of FIG. 14 lookingfrom the right hand side into chips I8 and I6 in modules 1 and 2. Partof path 8 is shown in module 1 and part of path 7 is shown in module 2.Package connections P5 and P6 provide the signal on path 8 to chips I8and I16 of module 1 through very short stubs 140 and 142. Packageconnections P7 and P8 provide the signal on path 7 to chips I8 and I16of module 2 through very short stubs 144 and 146. Note that paths 7 and8 may be essentially on the surface of modules 1 and 2 or may be underthe surface of modules 1 and 2, but are shown inset for convenience indrawing. Paths 8 and 7 do not have to extend as high up into modules 1and 2 as is shown in FIG. 15. For example, they might extend just to thelevel of the package connections or above that level. OMTs could havebeen illustrated in FIG. 15.

In the case of chips I7 and I15 in module 1, path 7 has a loop thatlooks similar to path 8 in FIG. 15 except as follows. In FIG. 15, path 8is looped between two connector contacts M12 and M4 and is coupled topackage connections P5 and P6 between M4 and M12. The loop for path 7between 17 and 115 does not go between two connector contacts. Rather,in the case of I7 and I15, path 7 has a loop between I7 and I15 thatcouples package connections for I7 and I15, but it loops between M11 andsome interface to the OMT. This may be similar to or different from thatshown for path 1 in FIG. 3. The paths in the module each couple topackage connections. See FIG. 26 showing portions of routings for path 1and path 2.

FIG. 16 shows a front view of module 1 of FIG. 14. FIG. 16 illustratesOMTs coupled between chips I2, I4, I5, and I7 and connector connectionsCpath2 Vref, Cpath4 Vref, Cpath5 Vref, and Cpath7 Vref, respectively.Connector connections Cpath2 Vref, Cpath4 Vref, Cpath5 Vref, and Cpath7Vref interfaces with connectors that carry references signals (power,ground, or some other voltage depending on the implementation) for thesignals on paths 2, 4, 5, and 7, respectively. Merely as an example,there may be a 1:1 or 2:1 ratio of signal to reference conductors. Sinceconnector connections Cpath2 Vref, Cpath4 Vref, Cpath5 Vref, and Cpath7Vref are used whether or not there is OMTs, there is a substantialsavings (e.g. ¼) in fingers, connector contacts etc. (Compare discussionfor FIG. 10.)

FIG. 17 provides an electrical impedance diagram for path 1 in FIG. 14.ZoMB1 and ZoMB2 are impedances for the motherboard traces of path 1 inthe positions shown. ZoMod11, ZoMod12, and ZoMod13 are impedances inmodule 1 in the positions shown, and ZoMod21, ZoMod22, and ZoMod23 areimpedances in module 2 in the positions shown. (Note there are N of eachof these.) CI1 and CI9 represent capacitances for chips I1 and I9 ofmodule 1, and CI10 and CI2 represent capacitances for chips I10 and I2of module 2. It may be desirable to make the impedance of ZoMB2 equalZoMB1. It may be desirable to make the effective impedances of thecombined C11, ZoMod11 and ½ of ZoMod12 equal to that of ZoMB1. Likewise,it may be desirable to make the effective impedances of the combinedC19, ½ of ZoMod12, and ZoMod13 equal to that of ZoMB1 and so forth withmodule 2. To compensate for CI1, the impedance of ZoMod11 and perhapsZoMod12 may be higher than ZoMB1. Likewise, the other impedances inmodules 1 and 2 may be higher to compensate for CI9, CI10, and CI2. Itmay not be practical to get the impedances exactly equal because of sizeor other expense constraints. Merely as an example, the impedances ZoMB1and ZoMB2 might be 39 ohms and the impedances ZoMod11, ZoMod12, andZoMod13 and ZoMod21, ZoMod22, and ZoMod23 might be 63 ohms. Variousother values might be used such as 30 and 60 ohms, 33 and 63 ohms, and50 and 100 ohms, to mention only some of the possibilities.

In FIG. 17, the impedance may increase as the path gets closer to thechip loads. For example, the impedance ZoMod11 could increase as it getscloser to chip I1. ZoMod11 could be higher than impedance in connector116. Merely as an example, ZoMod12 could be higher than that of ZoMod11and ZoMod13.

The paths of systems 90 and 100 and systems described below may beterminated on a single end or both ends (in the chipset as well as onthe OMT). The capacitance C illustrated in controller 92 for path 1 maybe about 2 pF. However, controller 92 and other controllers shown in thefigures are not limited to the details illustrated.

3. Systems Includings ECC Chips and/or Buffer Chips

FIG. 18 illustrates a motherboard 200 which may be similar to that ofFIG. 14 except that it also includes ECC chips and buffer chips bufferM1 and buffer M2 on modules 1 and 2.

The ECC chips connect through connector connections C27 and C28 and maybe of a well known type. The ECC chips may be positioned in differentlocations from those shown.

The buffer chips buffer M1 and buffer M2 may received address and/orcommand signals from controller 202 on a path including conductors 204(which has M conductors). The path splits from conductors 204 toconductors 206 and 208, with conductors 206 coupling to connectorconnections C25 and conductors 208 coupling to connectors connectionsC26. Buffers M1 and M2 may in turn provide the address and/or commandsignals on P conductors to chips I1-I8 and ECC chips if present. (Ofcourse, the inventions are not restricted to the use of a particularnumber of chips per module.)

The ECC chips and the buffer chips buffer M1 and buffer M2 may beterminated on the motherboard, on the module, on die, and/or in thecontroller. (They may be single or dual terminated chips). Rather thanterminate at the end of single path after two ECC chips (actually fourchips if consider ECC chips on other side of modules are included) asshown in FIG. 18, there could be one path to the ECC chip(s) on module 1with an OMT on module 1 and another path to the ECC chip(s) on module 2with an OMT on module 2.

In FIGS. 18-20, C25 and C26 represent multiple connector connections,and P27 and P28 represent multiple package connections. Conductors 204represent multiple conductors with ZoMB1 representing correspondingimpedances. Conductors 208 represent multiple conductors with ZoMB2representing corresponding impedances. There are multiple conductorsbetween C25 and P27 with corresponding impedances ZMod1, and there aremultiple conductors between C26 and P28 with corresponding impedancesZMod2. There are multiple Cb1s and Rb1terms in buffer M1 and multipleCb2s and Rb2terms in buffer M2. Controller 202 is not restricted to thedetails shown in FIGS. 19 and 20. In FIGS. 19 and 20, signalregeneration for distribution to I1-I8 and ECC, etc. is not shown.

FIG. 19 shows an example of how the path (204 and 206) betweencontroller 202 and buffer M1 can be terminated on the die of buffer M1,and how the path (204 and 208) between controller 202 and buffer M2 canbe terminated on the die of buffer M2. More particularly, buffer M1includes termination resistors Rb1terms and buffer M2 includestermination resistors Rb2terms. Because of the split to 206 and 208, itmay be desirable that the impedances ZoMB2 are twice the impedancesZoMB1. It may be desirable that the impedances ZoMod1 are twice theimpedances ZoMB1, and the impedances ZoMod2 are twice the impedancesZoMB1. It may be desirable that the effective impedances of Cb1 andRb1term are twice the impedances ZoMB1, and the impedances Cb2 andRb2term are twice the impedances ZoMB1. However, in practice these goalsmight not be practical because of size or other cost constraints.Therefore, ZoMB2, ZoMod1, ZoMod2, and the effective impedances of Cb1and Rb1term and the effective impedances Cb2 and Rb2term might be,merely as an example, less than twice ZoMB1. For example, ZoMB1 merelyas an example, might be 39 ohms and the other impedance values might be63 ohms. Of course, the inventions are not restricted to these impedancevalues and other values might be used such as 30 and 60 ohms, 33 and 63ohms, and 50 and 100 ohms, to mention only some of the possibilities.The impedances other than ZoMB1 and ZoMB2 do not have to equal eachother. For example, the impedances for Rb1terms do not have to equalthat of ZoMod1.

FIG. 20 shows an example of how the path (204 and 206) betweencontroller 202 and buffer M1 can be terminated off the die of buffer M1,but on module 1, and how the path (204 and 208) between controller 202and buffer M2 can be terminated off the die of buffer M2, but on module2. More particularly, buffer M1 includes a capacitive load Cb1 andbuffer M2 includes a capacitive load Cb2. Module 1 includes conductorimpedances ZoMod11 and ZoMod12 and termination resistors Rmod1term.Module 2 includes conductor impedances ZoMod21 and ZoMod22 andtermination resistors Rmod2term. It may be desirable that the impedancesZoMB2 are twice the impedances ZoMB1 (because of the split to 206 and208). It may be desirable that the effective impedances of Cb1, ZoMod11and ZoMod12 be twice the impedances ZoMB1, and resistances of RM1term betwice ohms of the impedances ZoMB1. It may be desirable that theeffective impedances of Cb2, ZoMod21 and ZoMod22 be twice the impedancesZoMB1, and resistances of RM2terms be twice the ohms of the impedancesZoMB1. The loads Cb1 and Cb2 may not be significant so that ZoMod11,ZoMod12, ZoMod21, and ZoMod22 may be close to the same as RMod1terms andRMod2terms. However, in practice these goals might not be practicalbecause of size or other cost constraints. Therefore ZoMB2, Rmod1terms,Rmod2terms, and the effective impedances Cb1, ZoMod11 and ZoMod12 andeffective impedances of Cb2, ZoMod21 and ZoMod22 might be, merely as anexample, less than twice ZoMB1. For example, ZoMB1 merely as an example,might be 39 ohms and the other values might be 63 ohms. Of course, theinventions are not restricted to these impedance values and other valuesmight be used such as 30 and 60 ohms, 33 and 63 ohms, and 50 and 100ohms to mentioned only some of the possibilities. The impedances otherthan ZoMB1 and ZoMB2 do not have to equal each other. For example, theimpedances ZoMod11, ZoMod12, ZoMod21, and ZoMod22 may be different thanRMod1terms and RMod2terms.

The above described schemes (e.g., 39 ohms for ZoMB1 and others being 63ohms) may have two advantages. First, it reduces impedance mismatch.Second, the higher impedance values can be higher or lower depending onphysical layer PCB routing feasibility. An advantage of having thetermination on the module external to the die is termination does nothave to be added to the die thereby reducing the silicon thermaljunction temperature risk. This is at the expense of some signalintegrity reduction in comparison to on die termination.

In some embodiments of FIGS. 18-20, a p-channel push current mode drivermay be used in controller 202 and elsewhere, although the inventions arenot so limited. For example, such a driver may include a node at whichthe conductors 204 meet the controller 202. A p-channel field effecttransistor (PFET) has a signal (such as a data signal D#) couple to itsgate. The pFET is coupled between the node and a current source. Thecurrent source is coupled between the PFET and a power supply forcontroller 202. An Ro between the node and ground may be greater than 5times the impedances of conductors 204. However, this ratio may belowered to improve the matching at the expense of voltage swing for theequivalent driver current. An advantage of this I/O type is primarilythe ability to decouple the receiver voltage supply from the drivervoltage supply. Other advantages are its high speed capability and itsability for the Ro to be high or low in comparison to the impedance ofthe path of conductors 202. Of course, the inventions are not limited tothese details.

4. Clocking

FIGS. 21 and 22 illustrate a clocking system that may be used in someembodiments of the inventions. The inventions are not, however,restricted to the details of the clocking schemes of FIGS. 21 and 22.Portions of modules 1 and 2 of FIG. 14 are used for purposes ofillustration.

Referring to FIG. 21, clocking for chips I1, I9, I2, and I10 in modules1 and 2 are shown. Similar clocking can be duplicated for the otherchips in modules 1 and 2. Data paths 1 and 2 are shown as in FIG. 14 toprovide a context. Path 1 is coupled to package connections P21, P22,P23, and P24. Path 2 is coupled to package connections P41, P42, P43,and P44. A clock signal Clk is provided by controller 92 on a path tochips I1, I9, I2, and I0 in modules 1 and 2. The clocking path includesconnector connections C60 and then splits to go to package connectionsP51 of chip I1 and P52 of chip I9 and to package connections P53 of chipI2 and P54 of chip I10 of module 1. The split clock path merges andproceeds to connector connections C61 of connector 116 and travels toconnector connections C62 of connector 118. The clocking path thensplits to go to package connections P55 of chip I1 and P56 of chip I9and to package connections P57 of chip I2 and P58 of chip I10 of module2. The split clock path merges and proceeds to connector connections C63of connector 118 and travels to controller 92, wherein it may terminate.

The clock signal may be differential and therefore there may be twoconductors and corresponding connections in the clock path. A low orfull voltage swing clock may be used.

The clock signal of FIG. 21 provides timing for the data signals onpaths 1 and 2. Through this technique, a single clock signal can be usedfor eight chips. It may be preferred, however, to have one clock signalfor reading through path 1 and writing through path 2 and another clocksignal for reading through path 2 and writing through path 1. FIG. 22provides such a system. In FIG. 22, clock signal Clk1 is the same asclock Clk in FIG. 21. Paths 1 and 2 are not shown to avoid clutter inthe drawings, but paths 1 and 2 in the system of FIG. 22 may be the sameas in FIG. 21. FIG. 22 also adds another clock path to carry a clocksignal Clk2, which is provided by controller 92 on a path to chips I1,I9, I2, and I0 in modules 1 and 2. The clocking path for Clk2 includesconnector connections C71 of connector 118 and then splits to go topackage connections P55 of chip I1 and P56 of chip I9 and to packageconnections P57 of chip I2 and P58 of chip I10 of module 2. The splitclock path merges and proceeds to connector connections C72 of connector118 and travels to connector connections C73 of connector 116. Theclocking path for Clk2 then splits to go to package connections P65 ofchip I1 and P66 of chip I9 and to package connections P67 of chip I2 andP68 of chip I10 of module 2. The split clock path merges and proceeds toconnector connections C74 of connector 116 and travels to controller 92,wherein it may terminated.

In FIG. 22, data is written through path 1 synchronously with Clk1 andread through path 1 synchronously with Clk2. Data is written throughpath 2 synchronously with Clk1 and read through path 2 synchronouslywith Clk 1. Accordingly, reading and writing for eight chips can occurwith only two clock signals (which may each be differential signals).

It may be desirable if the impedances of the clocking and data paths arematched such that clocking and data signals have close to the sameswitching speed through these paths. The impedances can be increased(e.g. 25 to 50 ohms or 40 to 60 ohms or some other values) with thesplits and return to the original value when the paths merge. Asmentioned, the data paths of FIG. 17 may also have stepped increases inimpedance with loaded sections near the chip loads having even higherimpedance. There may be a further higher impedance of for example 65ohms (in the 25 to 50 ohms case) for sections that correspond to theloaded sections in the data paths to match the data paths. That is, theclock paths including sections that correspond to the loaded sections ofdata paths and have corresponding increases in impedances in thesections corresponding to the loaded sections to obtain good matching.As mentioned, although doubling impedances may be desirable for onestandpoint, it may not always be practical from an overall systemviewpoint and other impedance values may be used.

5. Multiple Module Systems

The following discussion and figures describes and shows systems withtwo sets of modules on different paths. These systems have particularapplication to the server environment, but are not restricted to thisenvironment. As illustrated, there are two modules per set. However,there could be three modules or more per set and/or more than two sets.

Referring to FIG. 23, a system 300 includes a motherboard 304 whichsupports four modules connectors 312, 314, 316, and 318 into whichmodules 1, 2, 3, and 4 are inserted. Modules 1 and 2 are in one set andmodules 3 and 4 are in another set. Paths 1, 2, . . . 8 (which may bebi-directional data paths) are provided to modules 1 and 2 as shown andterminate on motherboard 304 (motherboard terminations (MBT)). Forexample, path PI travels between controller 308 and a MBT by way ofchips I1 and I9 in module 1 and chips I1 and I9 in module 2. Path 1 maydo a short loop through in module 1 between chips I1 and I9 in a mannersimilar to shown in FIGS. 15 and 26, so as to provide the signals onpath 1 to package connections of chips I1 and I9. Likewise, path 1 maydo a short loop through in module 2 between chips I1 and I9 in the samemanner and then terminate outside module 2 in a termination package orother termination resistors supported by motherboard 334. Paths 2, 3 . .. 8 may also have a similar short loop through arrangement in modules 1and 2.

Paths 9, 10 . . . 16 are provided to modules 3 and 4 as shown andterminate with MBT. Paths 9, 10 . . . 16 may also have a short loopthrough arrangement in modules 3 and 4 similar to that of FIGS. 15 and26.

In the illustrated embodiment, chips I1-I16 are designed to receive N/2data bits and paths P1-P16 each have N/2 lines. For example, if in thesystem of FIG. 14, N is eight, then N/2 may be four in the system ofFIG. 23. In that case, the systems of FIGS. 14 and 23 would have thesame number of data lines (16×4 =8×8). (ECC chips may add additionallines.) However, the N in FIG. 23 does not have to be the same as the Nin FIG. 14, and N/2 does not have to be four. The paths of FIG. 23 donot have to have N/2 lines.

Referring to FIG. 24, a system 330 includes a motherboard 334 whichsupports four modules connectors 342, 344, 346, and 348 into whichmodules 1, 2, 3, and 4 are inserted. Modules 1 and 2 are in one set andmodules 3 and 4 are in another set. Paths P1, P2, . . . P4 (which may bebi-directional data paths) are provided to modules 1 and 2 as shown andterminate on motherboard 334 (MBT). Paths P5, P6, . . . P8 (which may bebi-directional data paths) are provided to modules 3 and 4 as shown andterminate on motherboard 334 (MBT). The paths may terminated on themodule or on die, but that may require one of the modules to bedifferent than the other(s) or to have a selectable terminations on dieor on the module (described below).

In the illustrated embodiment of FIG. 24, chips I1-I16 are designed toreceived N data bits and paths 1-8 each have N lines, which is twice asmany as the paths of FIG. 23. However, since there are also one half thenumber of paths in FIG. 24 as in FIG. 23, the number of lines in FIG. 23is the same as in FIG. 24 (N×8=N/2×16) as in FIG. 23. In FIG. 24, thepaths provide data to four chips. For example, path 1 provides data tochips I1, I2, I10, and I9. There are many ways in which this can bedone. FIG. 25 illustrates one way. Referring to FIG. 25, path 1 passesadjacent to chip I1 and is coupled to a package connections P21 of chipI1, either directly or through a via. Path 1 extends to chip I2 ofmodule 1, where it is coupled to package connections P44, and so forthwith path 1 coupling to package connections P43 of chip I10 and P22 ofchip I9 in module 1. The path does not have to have this particularlayout. For example, it does not have to extend above the chips orextend in straight lines or at 90 degree angles.

The routing of FIG. 25 can be compared to that of FIG. 26. FIG. 26illustrates a portion of the routing of paths 1 and 2 in module 1. Path1 in module 1 includes a short loop through section to couple to packageconnections P21 and P22.

The ECC, buffering, clocking schemes, short term card, and selectable ondie or on module terminations described herein may be used in connectionwith the systems 300 and 330 and other systems described herein. Systems300 and 330 are not restricted to using only two modules per path, butcould use three or more.

6. Termination Card (Dummy Module)

The system of FIG. 14 may be used with two modules or with a module andtermination card, which is an example of a dummy module. A dummy moduleincludes a circuit board (substrate), without the chips of an activemodule, that is used in place of the active module. A reason to have adummy module is because it is less expensive than the module, but itcompletes the circuit as does the module. Referring to FIG. 27, anexample of a termination card is short termination card 360 which in theillustrated embodiment fits into slot 18 of system 100 (see FIGS. 14 and15). Short termination card 360 is referred to as short because it isnot as tall as the modules 1 and 2. Short termination card 360 isdesigned to pass or terminate signals from the paths or other signals(e.g., ECC, buffering, clocking schemes or other features mentioned inthis disclosures) that may be present just as if it were module 2.

FIG. 28 illustrates a schematic plan view of short term card 360 havingfirst and second sides 362 and 364 connected to connector 118. Firstside 362 includes finger groups FG1, FG2 . . . FG8. Second side 364includes finger groups FG9, FG10, FG11, and FG12. In either FIG. 27 orFIG. 28, there may be additional finger groups if needed (for example,in the case in which there are additional paths or if the module withchips include ECC chips or a buffer).

As can be seen through comparing FIG. 28 with module 2 in FIG. 14, inthose paths for which there are no on module terminations, shorttermination card 360 fills the gap between connector contacts (e.g.,between M16 and M24 and FG1 and FG9). In the case in which an OMT isinvolved, it is not necessary to pass conductors through the module.Rather, a module could be on the same side as the path is received. Forexample, in FIG. 14, in module 2, conductors of path 1 pass between Ml 8and the other side of the module to couple to an OMT. Short terminationcard 360 could have a similar routing with the OMT on the same side asthat of module 2 (as shown in FIG. 27), or it could have the OMT on thesame side as M18 and not have to pass conductors across the module.

FIG. 27 shows possible locations of OMTs on module 1 and short term card360, although the inventions are not restricted to these locations. Anysuitable locations on the module is acceptable for the OMTs. Note thatin the case of module 1, the OMT is for a path other than path 8. In thecase of short term card 360 and module 1, only one of the OMTs is shown.

FIG. 29 shows an alternative short term card 368 in which the OMTs areon the second side 364 coupled to finger groups FG2, FG4, FG5 and FG7 byconductors.

In some embodiments, some OMTs could be on side 362 and other OMTs couldbe on side 364. This could be with a routing like that shown in FIGS.14, 28, and 29, or with a different routing in which some of the OMTs onthe module where on one side and some on the other.

7. Short Stub Created from a Path Loop in the Module and Selectable onDie Terminations

FIGS. 30-34 illustrate two independent aspects of the inventions (1) ashort stub created from a path loop in the module and (2) selectable ondie terminations. These aspects are presented together, but they may beused independently.

Note that in FIGS. 14 and 15, path 8 loops between package connectionsP5 and P6. By contrast, in FIG. 30, path 8 in a system 380 extends onlypartly into module 1 and couples to a short stub 386 which in turncouples to package connections P5 and P6 either directly or through avia. Path 8 has no loop in module 2 but rather extends all the way topackage connections P11 and P12 (or to a via that connects to P11 andP12). FIG. 31 shows a route for path 7 in which a short loop in module 2connects to a short stub 388 and no loop is in module 1. The loop inpaths 7 and 8 can be longer or shorter than shown (note that in FIG. 31,the loop in path 7 is shorter than that of path 8 in FIG. 30).

In a separate invention, in FIGS. 30 and 31, modules 1 and 2 each haveselectable on die terminations. In the example of FIGS. 30 and 31, theon die terminations are disabled in chips I8 and I16 of module 1 andchips I8 and I16 of module 2. The on die terminations are enabled inchips I7 and I15 of module 1 and I7 and I15 of module 2. The chips withthe enabled on die terminations may be the same as those in the modulewith OMT in FIG. 14. Accordingly, for some paths, module 1 will haveenabled on die terminations and module 2 will have disabled on dieterminations. For other paths, module 1 will have disabled on dieterminations and module 2 will have enabled on die terminations.

A circuit 400 in FIG. 32 is one example and a circuit 440 in FIG. 33 areexamples of circuits that can be used selectively enable or disable ondie terminations. Various other circuits could be used and theinventions are not restricted to these details of circuits 400 and 440.For example, the R-termination elements could be pull down rather thanpull up. Circuits 400 and 440 include an R-termination network 404 whichare illustrated in FIG. 34. Referring to FIG. 32, R-termination network404 includes X number of R-termination elements R-term 1, . . . R-termX. Depending on the implementation, X may be as low as less than 5 tomore than 100. Each element includes transistors T1, T2, and T3. TheR-term elements are controlled by an active R-term on/off selectioncircuit 408 through multiplexers 412-1 . . . 412-X and a linearizedactive R-term network bias circuit 410. In the illustrated circuit 400,the “1” value of multiplexers 412-1 . . . 412-X is tied to Vcc (but itmay be provided by bias circuit 410). The “0” value of multiplexers412-1 . . . 412-X is provided by bias circuit 410 (but it may be tied toground). That is, in the illustrated circuit 400, the “0” value mightnot be at ground to control how much transistors T1 and T2 are turnedon. Some feedback could be used to compensate for temperature, processvariations etc. Configurable driver 414 includes pre-driver swingcontrol circuit 416 and driver bias circuit 418. In FIG. 32, network 404is between power and data node 430 and driver 414 is between data node430 and ground. That is, the termination is to the power supply voltagenode. Alternatively, network 404 could be between node 430 and groundand driver 414 could be between the power supply node and ground. Notethat the system can have only one or more than one power supply andground voltage values.

In FIG. 34, system 380 includes a controller 384 which is coupled tomodules 1 and 2. Modules 1 and 2 and connectors 116 and 118 are similarto those in FIG. 15, except that the chips include selectable on-dieterminations instead of OMTs. Path 8 is illustrated. In FIG. 34, inmodule 1, chips I8 and I6 have on die terminations disabled and inmodule 2, chips I7 and I15 are on die terminations enable. Accordingly,in each of chips I8 and I16, selection circuit 408 select each ofmultiplexers 412 to provide the “1” value to turn off each R-termelement (R-term 1 . . . R-term X). Drivers 414 may also be turned off.By contrast, in chips I7 and I15, driver 414 is on and selection circuit408 selects at least one of the R-term elements of R-termination network404. The number of R-term elements selected and perhaps the “0” valueare controlled to give a desired impedance level, described next. Theremay be one or more than one selection circuit 408 and one or more thanone bias circuit 410.

Referring to FIG. 33, circuit 440 is similar to circuit 400 except asshown. For example, network bias circuit 410 controls the gates of FETsT1-1 and T2-1 . . . T1-X and T2-X. Multiplexers 412-1 . . . 412-X haveinputs tied to power and ground. The drains of T1-1 and T2-1 . . . T1-Xand T2-X are tied together.

Referring to FIG. 34, impedance ZoMB1 represents the impedance on path 8on the motherboard between controller 384 and connector 116 andimpedance ZoMB2 represents the impedance on the motherboard on path 8between connector 116 and connector 118. Impedances ZoMod11 and ZoMod13are impedances of path 8 in module 1 and ZoMod12 is the impedance ofstub 386 (see FIG. 30). Impedance ZoMod2 is the impedance of path 8 inmodule 2. It may be desirable if the impedances of ZoMod11, ZoMod12, andZoMod13 were larger than that of ZoMB1 and ZoMB2. Merely as an example,assume ZoMB1 and ZoMB2 were 39 ohms. The impedance of ZoMod11, ZoMod12,and ZoMod13 might be 63 ohms to, for example, compensate for chipcapacitance. Of course, other values (such as those listed above) couldbe used for tradeoffs with expense, board layout, and performance. Itmay be desirable if the impedances of the enabled on-die terminations(R-termination networks 404) in chips I7 and I15 were about twice thatof ZoMB1 and ZoMB2. For example, if ZoMB1 and ZoMB2 are 39 ohms, thenabout 78 ohms might be chosen for network 404. Of course, other valuesmight be chosen and the inventions are not restricted to these values.ZoMod2 may be the same as that of ZoMB1 or it could be higher, forexample to compensate for chip impedance.

In each of the systems described herein, an attempt can be made toincrease impedance gradually or through steps. For example, in FIG. 34,the impedance might increase from ZoMB1 to MoMod11 to ZoMod12 and thendecrease from ZoMod12 to ZoMod13 to ZoMB2. The trace sections near thechips may have higher impedance than those trace sections further fromthe chips.

As illustrated, system 380 is a dual termination system in thatcontroller 384 includes an R-term element 446 and a driver 414. Theremay be a separate R-term element for each line of path 8. Rather thenuse a single R-term element for termination, more than one element maybe used.

The on-die termination feature is not restricted to use with the shortstubbed systems of FIGS. 30 and 31. It may be used in connection withsystems with short loop through configurations (such as in FIG. 15). Inthat case, in FIG. 34, the impedances in module 1 would be like thoseshown in module 1 of FIG. 17. Further, the circuit of FIG. 32 could beemployed in connection with terminations in the chip package or on themodules.

8. Single Side Embodiments

FIGS. 11-34 illustrate systems and features thereof for dual sidedmodules (chips on both sides of the module). The invention, however, isnot restricted to use with dual sided modules, but rather could also beused with single side modules. FIG. 35 illustrates routing for a systemsimilar to that of FIG. 14 except that single side modules are usedrather than dual sided modules. FIGS. 36 and 37 show to routing pathsfor path 8 to terminate on an OMT through package connections P80.Various other positions of the OMT could be used. FIG. 35 showsconnector contacts being on both sides of the connector module slots (16and 18). Alternatively, they could all be on the front side of themodule slots.

9. Additonal Information and Inventions

The illustrations described above show only two modules. However, morethan two modules per path may be used. For example, FIG. 38 shows asystem 480 with a controller 482 which includes modules 1, 2, and 3.Module 3 is a module that from a path point of view acts as a bridgebetween modules 1 and 2. Accordingly, in any of the FIGS. 1-37, modules1 and 2 may be the same, with module 3 acting as a bridge betweenmodules 1 and 2. Alternatively, there could be different routings formodules 1 and 2 when there is a module 3.

In some embodiments of the inventions, a path does not go to twomodules, but stays on only one module and has on module termination onthat module. For example, FIG. 39 shows a system 490 which includes amotherboard 494 and paths as shown. The ECC and/or buffer chips may beused in any environment described herein.

The modules may be removable received into the module slots or may bemore permanently received by the connectors.

Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” or “other embodiments” means that a particularfeature, structure, or characteristic described in connection with theembodiments is included in at least some embodiments, but notnecessarily all embodiments, of the invention. The various appearances“an embodiment,” “one embodiment,” or “some embodiments” are notnecessarily all referring to the same embodiments.

If the specification states a component, feature, structure, orcharacteristic “may”, “might”, or “could” be included, that particularcomponent, feature, structure, or characteristic is not required to beincluded. If the specification or claim refers to “a” or “an” element,that does not mean there is only one of the element. If thespecification or claims refer to “an additional” element, that does notpreclude there being more than one of the additional element.

Those skilled in the art having the benefit of this disclosure willappreciate that many other variations from the foregoing description anddrawings may be made within the scope of the present invention.Accordingly, it is the following claims including any amendments theretothat define the scope of the invention.

What is claimed is:
 1. A system comprising: first and second modules,the first module having a first group of chips and the second modulehaving a second group of chips; a circuit board including first andsecond module connectors to receive the first and second modules,respectively; a first buffer on the first module and a second buffer onthe second module; and a path including conductors in a first sectionthat splits into a second section and third section, wherein the secondsection couples to the first buffer and the third section couples to thesecond buffer, and wherein the first buffer provides signals receivedfrom the second section to the first group of chips and the secondbuffer provides signals received from the third section to the secondgroup of chips, and wherein impedances of the second and third sectionsare each at least 50% greater than impedances of the first section. 2.The system of claim 1, wherein the impedances of the second and thirdsections are each greater than 60 ohms and the impedances of the firstsection are less than 40 ohms.
 3. The system of claim 1, wherein thefirst module includes terminations for the path including the first andsecond sections and the first buffer is in parallel with theterminations.
 4. A The system of claim 1, wherein the first moduleincludes first terminations for the path including the first and secondsections and the first buffer is in parallel with the firstterminations, and the second module includes second terminations for thepath including the first and third sections and the second buffer is inparallel with the second terminations.
 5. The system of claim 1, whereinthe first and second buffers each have impedances that are at least 50%greater than the impedances of the first section.
 6. The system of claim1, wherein the first buffer includes terminations for the path includingthe first and second sections and the second buffer includesterminations for the path including the first and third sections.
 7. Thesystem of claim 1, wherein the first buffer provides signals receivedfrom the second section to the first group of chips and the secondbuffer provides signals received from the third section to the secondgroup of chips.
 8. The system of claim 3, wherein the terminations eachhave impedances that are at least 50% greater than the impedances of thefirst section.
 9. The system of claim 7, wherein the signals includeaddress and command signals.
 10. The system of claim 8, wherein theimpedances of the terminations are each greater than 60 ohms and theimpedances of the first section are less than 40 ohms.
 11. A systemcomprising: first and second modules, the first module having a firstgroup of chips and the second module having a second group of chips; acircuit board including first and second module connectors to receivethe first and second modules, respectively; a first buffer on the firstmodule and a second buffer on the second module; a path includingconductors in a first section that splits into a second section andthird section, wherein the second section couples to the first bufferand the third section couples to the second buffer; and wherein thefirst buffer includes on die terminations for the path including thefirst and second sections, and the second buffer includes on dieterminations for the path including the first and third sections. 12.The system of claim 11, wherein impedances of the second and thirdsections are each at least 50% greater than impedances of the firstsection.
 13. The system of claim 11, wherein impedances of the secondand third sections are each greater than 60 ohms and impedances of thefirst section are less than 40 ohms.
 14. The system of claim 11, whereinthe first buffer provides signals received from the second section tothe first group of chips and the second buffer provides signals receivedfrom the third section to the second group of chips.
 15. The system ofclaim 11, wherein the signals include address and command signals.
 16. Asystem comprising: first and second modules, the first module having afirst group of chips and the second module having a second group ofchips; a circuit board including first and second module connectors toreceive the first and second modules, respectively; a first buffer onthe first module and a second buffer on the second module; a pathincluding conductors in a first section that splits into a secondsection and third section, wherein the second section couples to thefirst buffer and the third section couples to the second buffer; andwherein the first module includes first terminations for the pathincluding the first and second sections and the first buffer is inparallel with the first terminations, and the second module includessecond terminations for the path including the first and third sectionsand the second buffer is in parallel with the second terminations. 17.The system of claim 16, wherein the impedances of the second and thirdsections are each greater than 60 ohms and the impedances of the firstsection are less than 40 ohms.
 18. The system of claim 16, wherein thefirst and second terminations each have impedances that are at least 50%greater than the impedances of the first section.
 19. The system ofclaim 16, wherein the impedances of the first and second terminationsare each greater than 60 ohms and the impedances of the first sectionare less than 40 ohms.
 20. The system of claim 16, wherein the firstmodule includes first terminations for the path including the first andsecond sections and the first buffer is in parallel with theterminations, and the second module includes second terminations for thepath including the first and third sections and the second buffer is inparallel with the second terminations.
 21. The system of claim 16,wherein the first buffer provides signals received from the secondsection to the first group of chips and the second buffer providessignals received from the third section to the second group of chips.22. The system of claim 16, wherein the signals include address andcommand signals.
 23. The system of claim 16, wherein the circuit boardis a printed circuit board and a motherboard.